LAUSR

Abstract The Taiwan Strait (TWS), connecting the South China Sea and the East China Sea, is affected by complex land-sea interactions as well as by human activities on both sides of the Strait. In such a subtropical strait ecosystem, phytoplankton communities exhibit spatiotemporal variations that are complex and dynamic compared with those of classical coastal-oceanic systems. Long-term measurements and reliable analytical methods are required to clarify the processes and mechanisms associated with these complex changes. Here, we use decadal-scale field observations in the TWS (25 cruises during 2004–2018) to show that the realized niches of phytoplankton groups enable to explain the spatiotemporal variations of the phytoplankton communities in response to the different water masses in the TWS during the Northeast (NE) and Southwest (SW) monsoons. Diatoms, prasinophytes, and chlorophytes are characterized by the wide breadths of their temperature, salinity, and nutrient niches in both monsoons. They not only dominate in coastal water during the NE monsoon, but also flourish in upwelled water and plume water during the SW monsoon. Synechococcus and haptophytes were characterized by relatively high temperature, high salinity, and intermediate nutrient concentrations, and dominated in mesotrophic offshore water. Cryptophytes and dinoflagellates performed well in the coastal water influenced by the China Coastal Current during the NE monsoon due to their low-temperature, low-salinity, and high-nutrient niches. Prochlorococcus, a specialist with high-temperature, high-salinity, low-nutrient niches, is high in water affected by the Kuroshio intrusion in both monsoons. These results facilitate prediction of the responses of a subtropical strait ecosystem influenced by human activities and climate change. Multiple stresses would be especially problematic for phytoplankton such as cryptophytes and Prochlorococcus that are sensitive to changes of temperature and nutrient concentrations.